To play the game, the players must compete with each other to build a complete city with 8 districts (though the number can be reduced to shorten the game's play time). The game ends when a player completed their city, and the player with the highest amount of points wins.
The players begin each round by selecting 1 of 8 character cards, each possessing a particular skillset:
- Assassin: Kills another character, and the murdered character would not have a turn in that round.
- Thief: Steals all the gold from another character except the assassin.
- Magician: Can trade hands with another player, or discard any number of cards from hand and taking a new selection from the deck.
- King: Receives the "king" token, which means they would initiate the character selection and choose the first character card. They would also receive an extra gold for every yellow-coloured they build.
- Bishop: Players receives an extra gold for every blue-coloured district they build. Also, their districts cannot be destroyed by the warlord.
- Merchant: Receives 1 extra gold per turn. Also gains an extra gold for each green-coloured district they build.
- Architect: Player can draw 2 extra cards. They can also build up to 3 districts at once.
- Warlord: Can destroy another player's district (by paying the cost of the particular district minus 1 gold). Gains 1 extra gold for every red-coloured district they build.
At the start of the game, each players are given 4 district cards, and they play their turns by the order of the numbers of their character cards. During their turn, the players can take the following actions: draw 2 more district cards or collect two gold pieces, use their characters' skills and, if they have enough gold pieces, build a district (unless they were killed by the Assassins, during which they would not be able to do any of these). After each round, the characters are re-shuffled, and the cycle begins anew.
Each district cards have a specific point value, based on their building cost (i.e. a district that costs 5 gold to build would be worth 5 points, though the point would only be counted if you actually build them). Each cards are also given different colour codes (red, yellow, green, blue and purple), which can give the players coin benefits depending on the character cards they have. The purple cards are "special districts" which have their own unique powers. Accumulated gold will not be counted to the players' final score.
Three expansion packs have been released: the "Additional character set" edition, which provides a set new characters that can be swapped in with the old cast based on their numerical counterparts, the "Dark City" edition, which adds 14 new purple district cards, and the "Circus" edition, which features new "Action cards" that offers new tactical advantages.
Citadels provides examples of the following tropes:
- Brainwashed: The Witch is able to "bewitch" another character, and would assume their turn, being able to use the "Bewitched" character's special powers to the Witch's own advantage. The player whose character is "bewitched" gets the standard amount of resources (coins or cards) for themselves, but cannot use powers or build districts.
- Colour-Coded for Your Convenience: There are 5 different types of districts, each coded with a different colour and provides various advantages for the players. Having all 5 district types in your city will grant you 3 extra points at the end of the game.
- Royal/Noble districts (Yellow): Gives 1 extra gold for the King.
- Religious districts (Blue): Gives 1 extra gold for the Bishop.
- Trading districts (Green): Gives 1 extra gold for the Merchant.
- Military districts (Red): Gives 1 extra gold for the Warlord.
- Special districts (Purple): Various. Special benefits is inscribed in the individual district cards.
- Death Is Cheap: Since the characters are re-shuffled every round, anyone who has been killed by the Assassin will be "revived" in the next round.
- Low Fantasy: The selectable characters includes a Magician (and, from the expansion pack, a Witch and a Wizard), while one of the districts is called the School of Magic. Despite this, magic doesn't play a significant role in the gameplay (since the main focus of the game is to build cities and not RPG-based combat), and no non-human characters appear to exist in the game's setting.
- Medieval European Fantasy: The designs of the characters and districts combines Renaissance art-style and Standard Fantasy Setting costumes and locations.
- No Plot? No Problem!: Although the game provides a very elaborate setting with rather complicated rules about the characters' dynamics and district-based strategy, there's no actual plot/storyline beyond "build better districts than the other players to earn the most points by the end of the game". The characters don't even come with any backstory, since they are not meant to be individual persons, but a representative of various classes/occupations within a kingdom.
- Place of Protection: An interesting variation, as the protection comes from the character, rather than the actual place they are affiliated to, but if you are playing as the Bishop, your districts cannot be touched by the Warlord or Diplomat.
- Purple Is Powerful: The Special district cards, which gives some of the best advantages in the gameplay, are coded with purple.
- That One Rule: The Ballroom. If a player builds this and holds the crown (thus calling the order of turns) players must reply "Thank you, Your Excellency" or lose their turn.